Boiler water is the liquid phase of steam within a boiler. The term may also be applied to raw water intended for use in boilers, treated boiler feedwater, steam
Fire-tube boiler – Wikipedia
A fire-tube boiler is a type of boiler in which hot gases from a fire pass through one or (many) more tubes running through a sealed container of water.
Steam boiler sight glass: this article describes how to find & use the sight glass (or sight gauge) on a steam heating boiler to check or set steam boiler water level.